A pneumatic tire has historically been sought which has a reliable means of supporting the load of a vehicle upon the loss of the tire's inflation gas. Such a tire would allow the motorist to continue operating his vehicle for a limited distance before being required to change the tire. A tire having this "run-flat" feature would reduce the frequency of tire changes on high speed highways, thereby resulting in increased safety for the motoring public. Many devices have been proposed which will prevent the collapse of the tire upon its deflation. These devices are usually nonintegral with the tire and therefore difficult to mount, cumbersome, and generally not completely successful.
A more desirable approach to obtain a tire which will operate over short distances without the support of inflation gas is to construct the tire such that it will be self-supporting. The self supporting feature is achieved by employing relatively thick rubber reinforcement inward of the carcass ply in the middle sidewall area of the tire. The rubber reinforcement prevents the tire from collapsing completely and thus provides a limited driving range or run-flat capability after the loss of inflation.
U.S. Pat. Nos. 3,983,918 and 3,954,131 disclose tires having this inner sidewall reinforcement.
A requirement for successful performance of such a tire, both in the presence and absence of inflation gas, is that the inner sidewall material, which undergoes the most severe deflection, be made of an elastomeric material having a high modulus of elasticity and a low hysteresis. Natural rubber compositions have traditionally been preferred for applications requiring high modulus and low hysteresis. However, natural rubber, after prolonged exposure to heat, undergoes reversion, which means that the initial stiff high-modulus vulcanized material gradually softens and exhibits lower and lower modulus values approaching that of the original unvulcanized crude rubber, which ultimately are too low for satisfactory support of the weight by an uninflated tire. To prevent reversion of the natural rubber compound, large quantities of highly active antioxidants are used. These antioxidants, although effective, present other problems. The most effective antioxidants are staining and if a compound containing staining antioxidants is placed next to a white rubber compound, the antioxidant will migrate to the white rubber and cause discoloration. Since the inner sidewall compound is located directly under the white area of a white sidewall tire, staining is a serious problem. It is desirable to have a tire which will operate with and without inflation gas and not stain the white sidewall.